Optimum Design Of Stiffened Shell Structures For New Generation Launch Vehicle

Due to high specific strength and stiffness, stiffened shells have been widely used in the fuel tanks and interstages of launch vehicles. Lightweight design methodology of stiffened shells becomes unavoidable to guarantee the carrying capacity of launch vehicles. Firstly, two hybrid optimization methods based on equivalent stiffness model and exact model were proposed. Then, another efficient optimization method was developed for multi-variable optimization of stiffened shells. Subsequently, imperfection sensitivity of stiffened shells was investigated systematically, and Worst Multiple Perturbation Loads Approach (WMPLA) was studied. Futhermore, two concepts of hierarchical stiffened shells with low imperfection sensitivity were proposed, together with the corresponding optimization formulations considering imperfection sensitivity. Finally, several rapid design softwares based on ABAQUS were developed, taking the previous analysis and optimization experiences into account.The main works of this dissertation are given as follows:(1) Two hybrid optimization methods based on equivalent model and exact model were investigated. With regard to China’s new generation and future heavy-lift launch vehicles, due to the high computational cost of post-buckling analysis for stiffened shells, a bi-step size-layout optimization framework was presented for uniform axial compression, while a concept of partition design was utilized for non-uniform axial compression, and then a surrogate-based hybrid optimization framework was further developed. Results indicated that the optimization benefits of these two methods were significant.(2) Motivated by the design requirement of liquid oxygen tanks in the boosters of launch vehicles under non-uniform axial compression during ascent, a surrogate-based optimization framework with adaptive sampling allowing for user’s preference was proposed. Based on variable category and various sensitivity indices, an adaptive sampling method allowing for user’s preference was further developed, by which design space could be reduced rationally, and thus optimization efficiency was improved. Results of a three-dimensional rigid frame showed that the optimization efficiency was improved significantly, and the optimum result was proved to be potentially close to the global optimum. Results of a liquid oxygen tank model indicated that a more superior design could be achieved with less computation cost. (3) Based on dimple-shape imperfections, which probably occurs in thin-walled structures of launch vehicles, WMPLA was proposed. Firstly, the sensitivities to imperfections with various forms were studied in detail for various types of stiffened shells. Then the influence of a single dimple-shape imperfection on the load-carrying capacity of cylindrical shells was investigated by varying the amplitudes, directions and positions of imperfections. Further, a combined dimpe-shape imperfection was developed to investigate the interaction of dimples and the reduction of the load-carrying capacity. Finally, WMPLA was presented to find the worst imperfection and lower bound of load-carrying capacity, aiming at providing a reference for improving knockdown factors of thin-walled structures in China’s new generation and future heavy-lift launch vehicles.(4) The concept of stiffened shells for low imperfection sensitivity was put forward, and two configurations of hierarchical stiffened shells with low imperfection sensitivity were proposed herein. Further, two optimization formulations considering imperfection sensitivity were also developed. For stiffened shells with hyperbolic generatrix shape and bi-directional hierarchical stiffened shells, and the mechanical essences of their low imperfection sensitivities were studied. Numerical results showed that the outward hyperbolic generatrix shape could improve the load-carrying capacities of both perfect and imperfect elastic-buckling stiffened shells, while for the plastic-buckling stiffened shell, the inward hyperbolic generatrix shape could increase the collapse loads of perfect structures in a small amplitude, and the outward hyperbolic generatrix shape could improve the load-carrying capacities of imperfect structures significantly, reflected as the low sensitivity to global-and local-pattern imperfections. Numerical results also indicated that the existence of major stiffeners in hierarchical stiffened shell had the ability to restrict the development of the initial out-of-plane deformation, thus the evolution to the global-pattern deformation was slowed down, and the imperfection sensitivity was further decreased. Furthermore, two optimization formulations considering imperfection sensitivity were proposed to search for the hierarchical optimum design. Finally, the effectiveness of the proposed optimization formulations was validated.(5) Strength and stability analyses software of perfect and imperfect stiffened shells was developed, together with another four design-aided softwares, including quality verification software of FEM model based on ABAQUS, etc., which formed a series of modular softwares for the modeling of whole launch vehicle and missile, and also provided a convenient design tool for structural designs of launch vehicle and missile. The above softwares have been successfully utilized in the developments of new launch vehicles and missiles in China. The research of this dissertation was supported by the Excellent Doctoral Dissertation Sustentation Fund of Dalian University of Technology, the973program (2014CB049000,2014-2019), the National Natural Science Foundation of China (90816025,2009-2012), the Advanced Research Project of Equipment Department (625010345,2011-2014), the Fundamental Research Funds for the Central Universities (DUT11ZD(G)04,2011-2012) and several national defense projects. The financial contributions are gratefully acknowledged.